Structure, ordering, and bonding of half antiperovskites: PbNi3/2S and BiPd3/2S

In a model study the crystal structures of shandite (Pb₂Ni₃S₂), parkerite (Bi₂Ni₃S₂) and their Pd homologues are investigated in terms of ordered half antiperovskites AM_3/2S (A = Pb, Bi; M = Ni, Pd). This addresses fundamental questions on the structural relations, ordering and chemical bonding. From crystal structure investigations a new cubic parkerite variant is presented for Bi₂Pd₃S₂ that fits in an ordering model equivalently to shandite and parkerite. Type–antitype relations to ordered oxygen deficit perovskites are presented. With the relation to the superconductor Ni₃MgC a model is deduced that provides the complete crystal structure and symmetry in terms of the Ni and Pd ordering in antiperovskite superstructures. Therein a systematic ab initio investigation on the relative stability of shandite and parkerite structures is carried out for the first time. From the DFT modelling results the preferences of the ordering variants and the distinct differences in the atomic coordination spheres are discussed. The bonding in the systems is investigated by site projected density of states and covalent bond energy calculations.